1. Field of the Invention:
This invention relates to a continuous extrusion molding method for preparing products of various crosssectional configurations by extrusion molding of thermosetting resins.
More specificially, the invention relates to a method for continuous molding of thermosetting resins which comprises extruding a thermosetting resin so that the resin has at the outlet end of an extruder a flowability corresponding to a needle penetration degree of at least 20 and immediately curing the extruded resin in a mold at a temperature of 80.degree. to 250.degree. C under a pressure not higher than 200 kg/cm.sup.2 for a residence time of 1 second to 5 minutes.
2. Description of the Prior Art:
Extrusion molding of thermoplastic resins such as polystyrene resins, polyethylene resins and vinyl chloride resins has heretofore been conducted broadly in various fields. However, uses of molded articles of thermoplastic resins are severely limited because they are inferior in heat resistance and mechanical strength.
Thermosetting resins have heretofore been used for a molding method for preparing molded articles of a relatively small size, such as compression molding and injection molding methods, or for a molding method of a very low manufacturing efficiency such as a hand lay-up molding method.
A method for extrusion molding of thermosetting resins according to the ram process is known in the art (see U.S. Pat. No. 3,461,490). In this known method, however, it is impossible to mold thermosetting resins by continuous extrusion because the molding operation has to be conducted intermittently. More specifically, when the final stroke of the ram is completed, the extrusion operation is stopped, and after the ram is returned to the start point, the material is supplied and the molding operation is initiated again. Accordingly, since the molding operation is stopped when the ram is returned to the starting point, it is impossible to conduct the molding operation continuously, and since the molding pressure attained when the ram is advancing differs greatly from the pressure imposed when the ram is retreating, both the kind of molded product obtainable and the starting thermosetting resins are severely limited.
There is also known a method comprising plasticizing a thermosetting resin by employing an extruder of the screw type, feeding the plasticized material to a transfer pot, and casting the material into a mold from the transfer pot by means of a compression or injection molding apparatus different from the extruder used for plasticization, to thereby effect the compression or injection molding (see U.S. Pats. No. 3,233,288, No. 3,278,992 and No. 2,746,089). In such known method, however, the extruder is used only for plasticization of a thermosetting resin and attainment of safe feeding of the resin material, and the molding pressure is given by a different apparatus. In other words, in such known method, though the extruder is used for feeding of the resin material, the molding mechanism is substantially the same as that adopted in the conventional compression or injection molding.
In each of these known methods, the molding operation is conducted intermittently, and hence, it is impossible to perform extrusion molding continuously.
As means for overcoming such defects involved in the conventional methods, there has been proposed a method for drawing molding of thermosetting resins. In this method, since the resin material must resist the drawing force, use of a reinforcer such as a glass fiber roving is indispensable, and limitations are imposed on the kind of the resin material, the amount of a filler or other additive, and the like, with the result that application fields of final molded products are naturally limited.
Further, thermosetting resin compositions such as disclosed in, for example, U.S. Pat. No. 3,562,201 have been used for the conventional transfer molding method or extrusion molding method for preparing molding articles of a small size, but it is impossible to perform the extrusion molding continuously with use of such thermosetting resin composition.
As is seen from the foregoing, no method capable of continuously extrusion molding ordinary thermosetting resins has been known in the art.